As blue-violet semiconductor lasers are put into practice, a Blu-ray Disc (hereinafter, called as BD) as a high-density and large-capacity optical information recording medium (hereinafter, also called as an optical disk) having substantially the same size as CD (Compact Disc) and DVD (Digital Versatile Disc) is also put into practice. BD is an optical disk configured such that information is recorded or reproduced on or from an information recording surface of an information recording medium having a light transmissive layer thickness of about 0.1 mm, with use of a blue-violet laser light source configured to output laser light of about 400 nm-wavelength and with use of an objective lens having a numerical aperture (hereinafter, also called as NA) of about 0.85. In the present specification, a light transmissive layer is a layer between a surface of an information recording medium and an information recording surface.
There are known many kinds of single objective lenses or so-called compatible objective lenses configured to record or reproduce information with respect to optical disks of different types having light transmissive layer thicknesses different from each other.
For instance, patent literature 1 or patent literature 2 discloses a compatible objective lens configured such that spherical aberration resulting from a difference in light transmissive layer thickness between optical disks of different types is corrected with use of a difference in the light source wavelength by forming a diffraction structure (an optical path difference providing structure) on the objective lens.
FIG. 27 is a diagram illustrating a configuration of a conventional compatible objective lens. As illustrated in FIG. 27, a compatible objective lens disclosed in patent literature 1 is provided with a center area CN (an inner circumference area) formed on at least one optical surface of the compatible objective lens, an intermediate area MD (an intermediate circumference area) formed on the periphery of the center area CN, and a peripheral area OT (an outer circumference area) formed on the periphery of the intermediate area MD. The center area CN is an area including the optical axis of the objective lens. The center area CN, the intermediate area MD, and the peripheral area OT are concentrically formed on one optical surface of the objective lens around the optical axis thereof.
The center area CN is formed with a first optical path difference providing structure, the intermediate area MD is formed with a second optical path difference providing structure, and the peripheral area OT is formed with a third optical path difference providing structure. The first to third optical path difference providing structures are e.g. a diffraction structure including concentric ring zones around the optical axis. The optical path difference providing structure is roughly classified into a blazed structure and a step structure. The blazed structure including the optical axis has a sawtooth shape in section, and the step structure including the optical axis has such a sectional shape that plural step portions are formed. The peripheral area OT may be a refractive surface.
The center area CN of the compatible objective lens is a shared area between a first information recording medium, a second information recording medium, and a third information recording medium for use in recording or reproducing information with respect to the first information recording medium, the second information recording medium, and the third information recording medium. The information recording medium is e.g. BD, with respect to which information is recorded or reproduced by an objective lens having NA of about 0.8 to 0.9. The second information recording medium is e.g. DVD, with respect to which information is recorded or reproduced by an objective lens having NA of about 0.60 to 0.67. The third information recording medium is e.g. CD, with respect to which information is recorded or reproduced by an objective lens having NA of about 0.45 to 0.51.
Specifically, the compatible objective lens is configured to converge a first light flux of a wavelength λ1 passing through the center area CN on an information recording surface of the first information recording medium, converge a second light flux of a wavelength λ2 (λ2>λ1) passing through the center area CN on an information recording surface of the second information recording medium, and converge a third light flux of a wavelength λ3 (λ3>λ2) passing through the center area CN on an information recording surface of the third information recording medium.
Further, it is conceived that the first optical path difference providing structure formed on the center area CN is configured to correct spherical aberration generated by a difference between a thickness t1 of a light transmissive layer of the first information recording medium, and a thickness t2 (t2>t1) of a light transmissive layer of the second information recording medium, and a thickness t3 (t3>t2) of a light transmissive layer of the third information recording medium; and to correct spherical aberration generated by a difference between the wavelengths of the first light flux, the second light flux, and the third light flux, with respect to the first light flux, the second light flux, and the third light flux passing through the first optical path difference providing structure. It is conceived that spherical aberration generated by a difference between light transmissive layer thicknesses can be corrected by spherical aberration generated by a difference between wavelengths.
The intermediate area MD of the compatible objective lens is a shared area between the first information recording medium and the second information recording medium for use in recording or reproducing infatuation with respect to the first information recording medium and the second information recording medium, and is not used in recording or reproducing information with respect to the third information recording medium. Specifically, the compatible objective lens is configured to converge the first light flux passing through the intermediate area MD on the information recording surface of the first information recording medium, and converge the second light flux passing through the intermediate area MD on the information recording surface of the second information recording medium. On the other hand, the third light flux passing through the intermediate area MD forms flare on the information recording surface of the third information recording medium.
The peripheral area OT of the compatible objective lens is a dedicated area for the first information recording medium. The peripheral area OT is used in recording or reproducing information with respect to the first information recording medium, and is not used in recording or reproducing information with respect to the second information recording medium and the third information recording medium. Specifically, the compatible objective lens is configured to converge the first light flux passing through the peripheral area OT on the information recording surface of the first information recording medium. On the other hand, the second light flux passing through the peripheral area OT forms flare on the information recording surface of the second information recording medium, and the third light flux passing through the peripheral area OT forms flare on the information recording surface of the third information recording medium.
Next, an operation of a conventional optical head configured to record or reproduce information with respect to BD as the first information recording medium, DVD as the second information recording medium, and CD as the third information recording medium is described. FIG. 28 is a diagram illustrating a schematic configuration of a conventional optical head 100. The conventional optical head 100 is loaded with a DVD/CD compatible objective lens 108a for use in recording or reproducing information on or from a DVD 70 and a CD 80, and a BD exclusive objective lens 108b for use in recording or reproducing information on or from a BD 60.
Blue-violet laser light of about 405-nm wavelength output from a blue-violet laser light source 101 is incident on a polarization beam splitter 102 as S-polarized light. The blue-violet laser light reflected on the polarization beam splitter 102 is converted into circularly polarized light by a quarter wave plate 103, and thereafter, is converted into substantially parallel light by a collimator lens 104. The blue-violet laser light converted into substantially parallel light is transmitted through a first mirror 105a, and then is reflected and bent by a second mirror 105b. The blue-violet laser light reflected on the second mirror 105b is converged as a light spot on the information recording surface of the BD 60 through the BD exclusive objective lens 108b. 
The blue-violet laser light reflected on the information recording surface of the BD 60 is transmitted through the BD exclusive objective lens 108b again, and is reflected on the second mirror 105b. The blue-violet laser light reflected on the second mirror 105b is transmitted through the first mirror 105a and the collimator lens 104, and thereafter, is converted into linearly polarized light on a path different from the outward path by the quarter wave plate 103. The blue-violet laser light converted into linearly polarized light is incident and transmitted through the polarization beam splitter 102 as P-polarized light, and then is incident and transmitted through a flat plate beam splitter 113 as P-polarized light. The blue-violet laser light transmitted through the flat plate beam splitter 113 is guided to a light receiving element 123 via an anamorphic lens 122, whereby a detection spot is formed. The blue-violet laser light detected on the light receiving element 123 is subjected to photoelectric conversion, followed by computation. The computation yields a focus error signal for use in tracking plane deviation of the BD 60, a tracking error signal for use in tracking decentering of the BD 60, and an information signal.
Red laser light of about 660-nm wavelength output from a dual-wavelength laser light source 111 is incident on the flat plate beam splitter 113 as S-polarized light. The red laser light reflected on the flat plate beam splitter 113 is transmitted through the polarization beam splitter 102, is converted into circularly polarized light by the quarter wave plate 103, and thereafter, is converted into substantially parallel light by the collimator lens 104. The red laser light converted into substantially parallel light is reflected and bent on the first mirror 105a. The red laser light reflected on the first mirror 105a is converged as a light spot on the information recording surface of the DVD 70 through the DVD/CD compatible objective lens 108a. 
The red laser light reflected on the information recording surface of the DVD 70 is transmitted through the DVD/CD compatible objective lens 108a again, and is reflected on the first mirror 105a. The red laser light reflected on the first mirror 105a is transmitted through the collimator lens 104, and thereafter, is converted into linearly polarized light on a path different from the outward path by the quarter wave plate 103. The red laser light converted into linearly polarized light is incident and transmitted through the polarization beam splitter 102 as P-polarized light, and is incident and transmitted through the flat plate beam splitter 113 as P-polarized light. The red laser light transmitted through the flat plate beam splitter 113 is guided to the light receiving element 123 via the anamorphic lens 122, whereby a detection spot is formed. The red laser light detected on the light receiving element 123 is subjected to photoelectric conversion, followed by computation. The computation yields a focus error signal for use in tracking plane deviation of the DVD 70, a tracking error signal for use in tracking decentering of the DVD 70, and an information signal.
Infrared laser light of about 780-nm wavelength output from the dual-wavelength laser light source 111 is incident on the flat plate beam splitter 113 as S-polarized light. The infrared laser light reflected on the flat plate beam splitter 113 is transmitted through the polarization beam splitter 102, is converted into circularly polarized light by the quarter wave plate 103, and thereafter, is converted into substantially parallel light by the collimator lens 104. The infrared laser light converted into substantially parallel light is reflected and bent on the first mirror 105a. The infrared laser light reflected on the first mirror 105a is converged as a light spot on the information recording surface of the CD 80 through the DVD/CD compatible objective lens 108a. 
The infrared laser light reflected on the information recording surface of the CD 80 is transmitted through the DVD/CD compatible objective lens 108a again, and is reflected on the first mirror 105a. The infrared laser light reflected on the first mirror 105a is transmitted through the collimator lens 104, and thereafter, is converted into linearly polarized light on a path different from the outward path by the quarter wave plate 103. The infrared laser light converted into linearly polarized light is incident and transmitted through the polarization beam splitter 102 as P-polarized light, and is incident and transmitted through the flat plate beam splitter 113 as P-polarized light. The infrared laser light transmitted through the flat plate beam splitter 113 is guided to the light receiving element 123 via the anamorphic lens 122, whereby a detection spot is formed. The infrared laser light detected on the light receiving element 123 is subjected to photoelectric conversion, followed by computation. The computation yields a focus error signal for use in tracking plane deviation of the CD 80, a tracking error signal for use in tracking decentering of the CD 80, and an information signal.
An astigmatism method with use of the anamorphic lens 122 or the like is employed in generating a focus error signal for use in tracking plane deviation of the BD 60, the DVD 70, and the CD 80. In the astigmatism method, a detection spot to be formed on the light receiving element 123 is detected with use of a four-part light receiving pattern.
The diameter of a detection spot to be formed on the light receiving element 123 is substantially proportional to the light flux diameter of laser light for recording or reproducing information with respect to each of the BD 60, the DVD 70, and the CD 80.
It is possible to determine the focal length and the numerical aperture of the DVD/CD compatible objective lens 108a and the BD exclusive objective lens 108b in such a manner that the light flux diameter of laser light for recording or reproducing information on or from the BD 60, and the light flux diameter of laser light for recording or reproducing information on or from the DVD 70 are substantially equal to each other. Further, it is also possible to determine the focal length and the numerical aperture of the DVD/CD compatible objective lens 108a and the BD exclusive objective lens 108b in such a manner that the light flux diameter of laser light for recording or reproducing information on or from the BD 60, and the light flux diameter of laser light for recording or reproducing information on or from the CD 80 are substantially equal to each other.
Next, there is described an operation to be performed by a conventional optical head provided with the compatible objective lens disclosed in patent literature 1 or patent literature 2, which is configured to record or reproduce information with respect to BD as the first information recording medium, DVD as the second information recording medium, and CD as the third information recording medium.
FIG. 29 is a diagram illustrating a schematic configuration of another conventional optical head 200.
Blue-violet laser light of about 405-nm wavelength output from a blue-violet laser light source 201 is incident on a polarization beam splitter 202 as S-polarized light. The blue-violet laser light reflected on the polarization beam splitter 202 is converted into circularly polarized light by a quarter wave plate 203, and thereafter, is converted into substantially parallel light by a collimator lens 204. The blue-violet laser light converted into substantially parallel light is reflected and bent on a mirror 205. The blue-violet laser light reflected on the mirror 205 is converged as a light spot on the information recording surface of the BD 60 through a compatible objective lens 208.
The blue-violet laser light reflected on the information recording surface of the BD 60 is transmitted through the compatible objective lens 208 again, and is reflected on the mirror 205. The blue-violet laser light reflected on the mirror 205 is transmitted through a collimator lens 204, and thereafter, is converted into linearly polarized light on a path different from the outward path by a quarter wave plate 203. The blue-violet laser light converted into linearly polarized light is incident and transmitted through the polarization beam splitter 202 as P-polarized light, and then is incident and transmitted through a flat plate beam splitter 213 as P-polarized light. The blue-violet laser light transmitted through the flat plate beam splitter 213 is guided to a light receiving element 223 via an anamorphic lens 222, whereby a detection spot is formed. The blue-violet laser light detected on the light receiving element 223 is subjected to photoelectric conversion, followed by computation. The computation yields a focus error signal for use in tracking plane deviation of the BD 60, a tracking error signal for use in tracking decentering of the BD 60, and an information signal.
Red laser light of about 660-nm wavelength output from a dual-wavelength laser light source 211 is incident on the flat plate beam splitter 213 as S-polarized light. The red laser light reflected on the flat plate beam splitter 213 is transmitted through the polarization beam splitter 202, is converted into circularly polarized light by the quarter wave plate 203, and thereafter, is converted into substantially parallel light by the collimator lens 204. The red laser light converted into substantially parallel light is reflected and bent on the mirror 205. The red laser light reflected on the mirror 205 is converged as a light spot on the information recording surface of the DVD 70 through the compatible objective lens 208.
The red laser light reflected on the information recording surface of the DVD 70 is transmitted through the compatible objective lens 208 again, and is reflected on the mirror 205. The red laser light reflected on the mirror 205 is transmitted through the collimator lens 204, and thereafter, is converted into linearly polarized light on a path different from the outward path by the quarter wave plate 203. The red laser light converted into linearly polarized light is incident and transmitted through the polarization beam splitter 202 as P-polarized light, and is incident and transmitted through the flat plate beam splitter 213 as P-polarized light. The red laser light transmitted through the flat plate beam splitter 213 is guided to the light receiving element 223 via the anamorphic lens 222, whereby a detection spot is formed. The red laser light detected on the light receiving element 223 is subjected to photoelectric conversion, followed by computation. The computation yields a focus error signal for use in tracking plane deviation of the DVD 70, a tracking error signal for use in tracking decentering of the DVD 70, and an information signal.
Infrared laser light of about 780-nm wavelength output from the dual-wavelength laser light source 211 is incident on the flat plate beam splitter 213 as S-polarized light. The infrared laser light reflected on the flat plate beam splitter 213 is transmitted through the polarization beam splitter 202, is converted into circularly polarized light by the quarter wave plate 203, and thereafter, is converted into substantially parallel light by the collimator lens 204. The infrared laser light converted into substantially parallel light is reflected and bent on the mirror 205. The infrared laser light reflected on the mirror 205 is converged as a light spot on the information recording surface of the CD 80 through the compatible objective lens 208.
The infrared laser light reflected on the information recording surface of the CD 80 is transmitted through the compatible objective lens 208 again, and is reflected on the mirror 205. The infrared laser light reflected on the mirror 205 is transmitted through the collimator lens 204, and thereafter, is converted into linearly polarized light on a path different from the outward path by the quarter wave plate 203. The infrared laser light converted into linearly polarized light is incident and transmitted through the polarization beam splitter 202 as P-polarized light, and is incident and transmitted through the flat plate beam splitter 213 as P-polarized light. The infrared laser light transmitted through the flat plate beam splitter 213 is guided to the light receiving element 223 via the anamorphic lens 222, whereby a detection spot is formed. The infrared laser light detected on the light receiving element 223 is subjected to photoelectric conversion, followed by computation. The computation yields a focus error signal for use in tracking plane deviation of the CD 80, a tracking error signal for use in tracking decentering of the CD 80, and an information signal.
An astigmatism method with use of the anamorphic lens 222 or the like is employed in generating a focus error signal for use in tracking plane deviation of the BD 60, the DVD 70, and the CD 80. In the astigmatism method, a detection spot to be formed on the light receiving element 223 is detected with use of a four-part light receiving pattern.
The diameter of a detection spot to be formed on the light receiving element 223 is substantially proportional to the light flux diameter of laser light for recording or reproducing information with respect to each of the BD 60, the DVD 70, and the CD 80.
The conventional compatible objective lens disclosed in patent literature 1 or patent literature 2 is such that the inner circumference area is used in recording or reproducing information on or from BD, DVD and CD, the intermediate circumference area is used in recording or reproducing information on or from BD and DVD, and the outer circumference area is used in recording or reproducing information on or from BD. Accordingly, the effective diameter (an aperture diameter) of BD corresponding to the diameter of the outer circumference area is largest, and the effective diameter (an aperture diameter) of CD corresponding to the diameter of the inner circumference area is smallest.
FIG. 30 is a diagram illustrating a state of a detection spot to be detected on a light receiving element in recording or reproducing information on or from BD. FIG. 31 is a diagram illustrating a state of a detection spot to be formed on the light receiving element in recording or reproducing information on or from DVD. FIG. 32 is a diagram illustrating a state of a detection spot to be detected on the light receiving element in recording or reproducing information on or from CD.
The effective diameters of BD, DVD, and CD with respect to a compatible objective lens are substantially the same as the respective corresponding light flux diameters. Therefore, as is obvious from a comparison between the conventional optical head 100 illustrated in FIG. 28, and the conventional optical head 200 illustrated in FIG. 29, in the optical head 200 incorporated with the compatible objective lens 208, as illustrated in FIG. 30 to FIG. 32, for instance, the detection spots to be formed on the light receiving element 223 are such that the detection spot diameter decreases in the order of BD, DVD, and CD. The detection spot diameter with respect to BD is largest, and the detection spot diameter with respect to CD is smallest.
Decreasing the diameter of a detection spot to be formed on a light receiving element makes it possible to decrease the size of a light receiving pattern on the light receiving element, whereby it is possible to reduce circuit noise, and to enhance frequency characteristics. Noise performance in recording or reproducing information on or from BD as a high-density optical disk is particularly important. Further, in recording or reproducing information at a high speed, frequency characteristics are also important. Accordingly, the diameter of a detection spot to be formed on a light receiving element is determined, taking into account of noise performance and frequency characteristics with respect to BD.
On the other hand, decreasing the diameter of a detection spot to be formed on a light receiving element may result in a decrease of the allowable amount of positional displacement of the detection spot resulting from positional displacement of an adhesively fixed laser light source or light receiving element, as the ambient environment or the like changes. This makes it difficult to secure a stable servo operation. Specifically, in an optical head incorporated with a compatible objective lens, the detection spot diameter with respect to DVD and CD is small, as compared with the detection spot diameter with respect to BD in any case. Therefore, the influence of positional displacement of a detection spot is particularly large with respect to CD.
Further, in a compact and thin optical head configured such that the size from the lower surface of an optical disk to the lower surface of the optical head is small, it is impossible to secure a large installation reference plane for various optical components constituting the optical head. As a result, positional displacement of the adhesively fixed optical components and light receiving element by a change in ambient environment or the like is large, as compared with a conventional large-sized optical head. In other words, in a compact and thin optical head, positional displacement of a detection spot by a change in ambient environment or the like is large.
As described above, in using a compatible objective lens in a compact and thin optical head, the influence of positional displacement of a detection spot may be intolerably large. Patent literature 1 and patent literature 2 fail to disclose a solution for the drawback.